Ships that operate in environmentally sensitive or highly regulated regions can be limited in the manner or time in which they can operate due to the noise generated by the ship. This occurs in the oil and gas field, where noise from mobile drilling ships limits drilling time due to the effect that the noise can have on migrating bowhead whales in Arctic regions. When bowhead whales are sighted, operations may be halted until they have safely passed, and this process can take many hours.
In addition, there is growing concern over the effect that shipping noise has on marine mammals. Some studies suggest that shipping noise can have a significant impact on the whale's stress hormone levels, which might affect their reproduction rates, etc.
Known attempts to reduce noise emissions from surface ships include the use of a so-called Prairie Masker, which uses bands of hoses that produce small freely-rising bubbles to mitigate ship's noise. However, small freely-rising bubbles are usually too small to effectively attenuate low-frequency noise. In addition, Prairie Masker systems require continuous pumping of air through the system, a process itself that produces unwanted noise, and also consuming energy and requiring a complex gas circulation system that is costly and cumbersome to the other operations of the ship. Finally, such systems cannot operate efficiently at large depths due to the challenges of delivering (e.g., pumping) sufficient amounts of air to significant depths.
One principle that is useful in approximating or understanding the acoustic effects of gas pockets in liquid (e.g., air pockets or bubbles or enclosures in water) is the behavior of spherical gas bubbles in liquid. The physics of gas bubbles is relatively well known and has been studied theoretically, experimentally and numerically.
FIG. 1 illustrates a gas (e.g., air) bubble in liquid (e.g., water). One model 10 represented by FIG. 1 for studying the response of gas bubbles is to model the bubble of radius “a” as a mass on a spring system. The effective mass is “m” and the spring is modeled as having an effective spring constant “k”. The bubble's radius will vary with pressures felt at its walls, causing the bubble to change size as the gas therein is compressed and expands. In some scenarios the bubble can oscillate or resonate at some resonance frequency, analogous to how the mass on spring system can resonate at a natural frequency determined by said mass, spring constant and bubble size according to a generalized Hook's law.
The movement of gas volumes enclosed by liquid can absorb ambient underwater sound or sound in an environment generally. These phenomena have been studied by others and by the present inventors and exploited for various purposes. For example, U.S. Pat. No. 8,636,101 and similar works are directed to scattering and damping of acoustic energy by a system of encapsulated air bladders tied to an underwater rigging. U.S. Pat. No. 7,905,323 and similar works are directed to studying the mechanism for absorption of acoustic energy in a gas filled cavity, generally to affect the acoustics of a room. U.S. Pat. No. 7,126,875 and U.S. Pat. No. 6,571,906 and similar works are directed to generating sound dampening bubble clouds from a bubble producing apparatus submerged under water. While U.S. Pat. No. 6,567,341 is directed to a boom with a gas injection system forming gas bubbles placed around a waterborne noise source to reduce the propagation of noise from the source.
Each of the above type of systems are intended to either cause an acoustic impedance mismatch or to cause resonance in a gas bubble or bubble cloud or gas-filled balloon so as to absorb and/or scatter acoustic noise energy present in the vicinity of the bubbles or balloons. The mechanics of these systems generally rely on the bubble-to-water interface to offer a resonator as described above to as to attenuate sound energy. Each of the above systems is of a given effectiveness and practicality, which may be suitable for some applications and may remain options available to system designers in the field.